Recently, wireless systems of UWB (Ultra Wide Band) (hereinafter simply “UWB”) in the microwave frequency band covering wideband signals equal to or more than 500 MHz, and UWB (hereinafter “millimeter wave UWB”) in the millimeter wave frequency band that is able to cover signals of a wider band, are being developed. Accompanying this, various communication schemes are proposed as near-field wireless communication schemes. Above all, UWB of high transmission capacity is wideband communication exceeding the transmission speed of 1 Gbps and using carrier frequencies in the band equal to or more than 1 GHz. However, the transmission distance of UWB is very short compared to wireless LAN and is between 3 to 10 meters. Consequently, as an application of UWB, for example, connecting equipment possessed by individuals in a wide band is possible (PAN: Personal Area Network).
However, to speak of PAN in a word, PAN is in an early period and has difficulty in finding a possible application of PAN at ease because equipment utilizing the application generally requires greater power for wideband characteristics and it is difficult to move carrying a lot of batteries for PAN.
An example of the application that utilizes PAN and requires wideband characteristics of UWB is a file swapping application. Its basic operation principle is disclosed in, for example, Patent Document 1. By this means, a small community can be established through random file swapping (described as message exchange in Patent Document 1) in a mobile environment. Further, although not disclosed in Patent Document 1, data to be exchanged is not limited to text document and file swapping for music, images, moving images and software is also possible. Particularly, according to UWB, the effective transmission speed is between 100 Mbps and several Gbps, and so some files of a large size can be swapped when people pass each other, so that file swapping is a suitable application for UWB.
WiMedia, which is the industry standard of UWB, has finished standardization of UWB using multiband OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme. The standardized scheme is a multi-beacon scheme using a beacon period. According to this scheme, synchronization between superframes is acquired by transmitting beacons from all terminals forming a network and swapping information between the terminals, and communication is performed by booking and maintaining a transmission and reception time slot (see Non-Patent Document 1).
However, it is very difficult to use this scheme in the crowd in which people pass each other, because this scheme employs a scheme of connecting the one entire beacon period to the other such that two groups passing each other acquire superframe synchronization. That is, according to this scheme, to be connected concurrently beacon periods between groups that pass each other, because beacon periods are connected concurrently between groups that pass each other, processing of connecting one entire beacon period to the end of the other beacon period is carried out and this processing requires time. For example, one group takes time to recognize the presence of the other group, time is also required to notify the presence of the other group to the superframe group entirely or partly (that is, a group of terminals that share the same superframe) and time is also required to make a move by coordinating timings. Further, until connection is finished, communication between terminals between groups is not completely impossible, but the time that can be utilized is reduced significantly. This configuration is not suitable for an application for performing file swapping upon passing, that is, for establishing frame synchronization in a short period and completing large volume data communication.
On the other hand, there is a scheme (tone signal synchronization scheme) for acquiring synchronization inside and outside superframe groups (that is, synchronization in a group and synchronization between groups) using an unmodulated narrowband tone signal instead of a wideband signal as in UWB. According to this scheme, by transmitting a narrowband signal having the same reachable range as the frame reachable range of UWB for a predetermined time length from each terminal, it is possible to acquire synchronization between superframe groups using the earliest start time of the narrowband signal as the boundary between the superframes. That is, is this scheme refers to a scheme for detecting between superframe groups a narrowband synchronized signal at one time in N laps of superframe during the superframe period or at the time the presence of other superframe groups is learned, and for synchronizing superframes at the time the narrowband synchronized signal is detected. Further, according to this scheme, wideband communication and narrowband communication are used in combination, so that it is possible to reduce power consumption.
This tone signal synchronization scheme does not need to transmit a beacon for synchronization, and, if reception checking step for surrounding devices is prepared, it is not necessary to modulate and transmit beacon signals from all nodes. Consequently, this scheme makes it possible to acquire synchronization (hereinafter, “resynchronization”) with another superframe group at ease. That is, when synchronization is acquired in the tone signal synchronization scheme, groups can join together quickly compared to a case of a simple beacon period scheme. Consequently, this scheme is suitable for file swapping upon passing.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-298406    Non-Patent Document 1: “Towards High Speed Wireless Personal Area Network-Efficiency Analysis of MBOA MAC,” Yunpeng Zang, etc., Internet URL: http://www.ecma-international.org/publications/standards/Ecma-368.htm